In an internal combustion engine with a pressure wave supercharger as the supercharging device, a starting valve or a starting butterfly valve acts to shut off the supercharge air line between the pressure wave supercharger and the inlet side of the engine during engine starting because the pressure wave process is still not running correctly after the engine fires, with the particular effect that the supercharge air still contains too much exhaust gas and the engine would be smothered by a supercharge of such an air/exhaust gas mixture. In this phase, therefore, the engine must be operated with air induced directly from the environment and, for this purpose, a weakly spring-loaded valve opened by the engine suction (a so-called snifter valve) is provided in the supercharge air line between the supercharge air butterfly valve and the induction manifold of the engine. As soon as the exhaust gas pressure before the cell rotor of the pressure wave supercharger is high enough to maintain a functioning pressure wave process, the supercharge air butterfly valve is opened and the engine is supplied, during further operation, with supercharge air generated by the pressure wave supercharger. The pivoting of the butterfly valve for the purpose of opening the charge air duct can be effected by a cylinder with a piston or diaphragm; the cylinder is subjected to the pressure difference between the exhaust gas pressure and the supercharge air pressure or between the latter and the ambient air pressure and has an active connection with the supercharge air butterfly valve.
In another known pistonless concept, the supercharge air butterfly valve is supported asymmetrically relative to the central axis of the supercharge air duct. As soon as the pressure wave process has come into operation after the engine is started, this butterfly valve is pulled away from its closed, locked position by the dynamic pressure of the supercharge air or by the pressure difference before and after the butterfly valve and then remains freely pivotable in the supercharge air flow during the duration of operation, its angular position adjusting itself in accordance with the dynamic pressure of the supercharge air flow. When the engine is shut down, the butterfly valve returns to its closed and locked initial position.
The above-mentioned concepts of supercharge air butterfly valves have been developed for pressure wave superchargers which have a constant gear ratio positive drive from the internal combustion engine, preferably by means of a belt drive. If the intention is that the full idling range of the engine should be satisfactorily covered with the butterfly valve fully opened, they have the disadvantage of requiring a particular geometrical design of the control edges formed by the air and gas ducts and of the pockets and other ducts and recesses in the gas and air casings. This design is not, however, the best possible one, particularly in the upper load range. Since the advantage of a pressure wave supercharger relative to an exhaust gas turbocharger consists precisely in a more rapid response of the engine to a demand for increased power in this operating range, this design represents a compromise at the expense of this range, which is the most important one for practical driving. A pressure wave supercharger designed for this range does, however, offer reserves within it and this makes it possible--for an engine of a given power--either to use a smaller pressure wave supercharger or to obtain better utilisation of the power potential of an engine using a pressure wave supercharger of a given size.
In the known concepts mentioned above, the supercharge air butterfly valve is either closed for too long a period after the engine starts, so that the engine power cannot be achieved in an optimum fashion, or it is continuously open to a greater or lesser extent during the running of the engine and is only completely closed during the starting phase; such a concept is not, therefore, feasible for a free running pressure wave supercharger. This is because, when the pressure wave supercharger is free running and driven by the exhaust gas flow alone, the speed of the pressure wave supercharger is still very low immediately after the engine fires, and it follows that with the supercharge air butterfly valve open, the recirculated exhaust gas quantity is very high so that the engine would immediately be smothered.
The concepts mentioned above also mean that emergency operation in the case of a damaged rotor of the pressure wave supercharger is only possible to a limited extent because, when the rotor is at rest, the closed supercharge air butterfly valve can be pulled away from its catch and pressed upwards by the large dynamic pressure of the exhaust gas before the butterfly valve; the exhaust gases can then enter the induction manifold of the engine. The reason for this is that, in contrast to exhaust gas turbochargers (in which the compressor and the turbine are separate) a short circuit between the high pressure gas duct and the high pressure air duct can occur in the case of pressure wave superchargers when the rotor is at rest; this leads to a direct passage of exhaust gas into the induction manifold of the engine and would lead to the engine being smothered.